Streamer cables are used in a variety of applications, usually involving remote sensing in water environments, for example, in fresh or saltwater bodies. The cables are typically used for seismic exploration surveys to gather information about subsurface geology, including oil and gas reserves. The cables usually contain one or more sensors for receiving signals present in the water. Often, the sensors include hydrophones for sensing acoustic signals. Individual streamer cables are often made up of a series of sections, which may be on the order of 75 meters long. In deployment, streamer cables, or arrays of several streamer cables, may be towed behind vessels, or deposited at fixed locations, such as ocean floor locations.
In a typical seismic exploration survey, one or more streamer cables are towed behind a tow vessel. The one or more streamer cables often form an array that may include a dozen or more parallel cables, each many kilometers in length. The tow vessel, or another vessel, may include an acoustic source for generating acoustic signals. The acoustic signals propagate through the water and interact with various structures in the water, on the ocean floor, and/or below the ocean floor. The interactions may produce reflections and/or refractions that may be sensed by the streamer cables and used to generate information about the reflecting or refracting structures. For example, an energy company may be able to locate areas beneath the ocean floor that are more likely to contain deposits of oil, gas and/or other minerals by examining the information developed from the acoustic reflections and/or refractions received by hydrophone sensors contained within a streamer cable. Often the acoustic signals used in seismic surveys are of a very low frequency, for example, between 3 and 150 Hz.
Streamer cables may also be used in other applications, such as surveillance applications. In some applications, one or more streamer cables are referred to as towed arrays. In an example surveillance application, a military vessel may tow or deposit a streamer cable containing sensors capable of detecting, for example, the noise generated by other vessels. Streamer cables in such surveillance applications may be capable of sensing acoustic energy over a much broader bandwidth than the 3-150 Hz frequency range discussed above.
Conventional streamer cables are liquid-filled. Liquid-filled streamer cables typically include an outer skin that houses the components of the cable including, sensors, strength members, wires, etc. After the streamer cable is configured, a liquid void-filler material, for example kerosene, is added to fill the void between the outer skin and the sensors, strength members, etc. The liquid void-filler typically provides acoustic coupling between the outer skin and the sensors. The specific liquid void filler material may be selected to adjust the overall density of the streamer cable and therefore affect its buoyancy.
Although they are widely used, liquid-filled streamer cables present several difficulties. First, because the liquid void-filler material may be mobile within the outer skin, mechanical energy resulting from the movement of the cable, tow vessel, and cable handling devices in the water may be acoustically coupled to the sensors by bulge waves in the cable. Additionally, the liquid void filler of liquid-filled cables may leak into the body of water when the outer skin is damaged. A liquid void-filler, such as kerosene may create an environmental hazard when leaked into a body of water. Also, leaked liquid void filler may be replaced in the streamer cable by water, which can degrade received signals and cause corrosion of the internal components of the streamer cable, especially in saltwater environments. As such, many energy companies and other users of streamer cables have begun to favor alternatives to liquid-filled streamer cables.
Solid streamer cables, i.e. streamer cables with solid void-fillers, have been developed in an attempt to address the problems of liquid-filled streamer cables. A common type of solid streamer cable includes a solid central core with sensors, skin, buoyant material, and other various components installed thereabout. Another type of solid streamer cable includes alternating sections of sensors and buoyant material. As an alternative to liquid-filled cables, solid streamer cables have superior leakage and bulge wave reduction qualities, but present other difficulties of their own.
For example, solid streamer cables suffer various problems related to buoyancy. The solid core and other solid materials in the cables typically have a density greater than that of seawater. Therefore, additional buoyant material, often hollow microbead material made of glass or foam, is placed in the streamer cable to reduce its overall density. Because the density of the microbead material is related to the quantity of air positioned within the microbeads, insuring consistent density throughout the microbead material may be costly and complicated.
Also, microbead material made of glass or foam tends to crush when subjected to excessive force, causing the overall density of the streamer cable to increase, and causing a loss of buoyancy. For example, if a solid streamer cable is handled roughly, or coiled at a small radius, microbead material may be crushed. Often tow vessels outfitted to handle and store liquid-filled cables must be refitted with new handling equipment, including coiling spools, etc., before the vessels may safely handle solid cables. Microbead material may also be crushed by water pressure if the streamer cable is operated beyond a certain depth. In extreme situations, solid cables with crushed microbead material may lose considerable buoyancy, and sink beyond recovery.
Solid streamer cables also suffer from noise problems due to shear wave energy. Because the solid materials in existing solid streamer cables, including glass microbead materials, have low compliance, energy from the motion of the cable in the water can be coupled through the solid materials to the sensors in the form of shear waves. Isolating the sensors from the shear waves presents a considerable challenge. Often, the sensors are placed in a rigid isolation structure or housing embedded in the solid void-filler material. The structure or housing may then be filled with a liquid void-filler for providing the necessary acoustic properties and negating the negative properties of the solid void-filler. Conventional streamer cables with liquid void-filler material typically do not contain such isolation structures for sensors. It can be appreciated that the necessity of isolating sensors within a cable, such as with isolation structures, adds complexity and expense, and also limits the potential configurations of the sensors and types of sensors that may be installed within the streamer cable.
Repairing existing solid streamer cables may also present certain difficulties. For example, it may be necessary to remove large sections of solid material to access damaged sensors or signal wire. It can be seen that replacing the sections of solid material of existing solid streamer cables without adversely affecting the signal qualities of the cable presents a considerable challenge.
Accordingly, there is a need for a streamer cable with a void-filler material that remains largely confined to the cable in the event of a failure of the outer skin. Also, there is a need for a solid void-filler material for retrofitting existing liquid-filled streamer cables. There is also a need for a streamer cable with a void-filler material that does not couple excessive shear wave or bulge wave energy to the sensors of a streamer cable.